Certain types of dynamoelectric machines, such as DC machines or synchronous machines, may include a field winding excited by a DC voltage. A synchronous machine is a doubly excited dynamoelectric machine wherein a rotor carries a field winding excited by a DC source. The current in the field winding, or field current, depends on an armature current and a grid voltage. An excitation controller provides the control of the field current by controlling a voltage across the field winding. The purpose of field current to is to produce a flux that links the stator winding for inducing an electromotive force (emf) for generating electrical power.
FIG. 1 illustrates a typical phasor diagram of a salient pole synchronous machine where Vt and Ia represent the stator voltage and stator current, respectively, during operation, δ represents a power angle, and φ represents a power factor angle. Id and Iq represent respective current components along direct axis (d axis) and quadrature axis (q axis). Xd and Xq represent the synchronous reactance along the d axis and the q axis, respectively. X1 represents leakage reactance, Em represents a total emf behind the leakage reactance, and E represents the total emf induced at a certain operating condition.
Over time, dynamoelectric machines, such as generators used to produce electricity, may become increasingly susceptible to field winding short conditions, such as shorted turns, that detrimentally affect operation of the machine. Conventionally, shorted turns are identified by performing periodic testing of the generator while operating at different loads. However, such testing may require the generator to be operated in a suboptimal mode or be taken off-line entirely while performing testing. On-line testing for shorted turns may be accomplished by monitoring a flux density provided by a flux probe inserted into an air gap between a rotor and an armature of the generator. Flux probe testing may require monitoring the generator over an extending period of time and over a wide variety of load ranges. However, a flux probe monitoring system may be prohibitively expensive and interpretation of the resulting flux density data is relatively subjective.